The present invention relates to silicon-germanium-based compositions comprising silicon, germanium and carbon (Si--Ge--C), methods for growing Si--Ge--C epitaxial layer(s) on a substrate, etchants especially suitable for Si--Ge--C etch-stops, and novel methods of use for Si--Ge--C compositions. In particular, the present invention relates to Si--Ge--C compositions, especially for use as etch-stops, related processes and etchants useful for microelectronic and nanotechnology fabrication.
The invention was made with U.S. Government support under (i) Phase I SBIR N00014-93-C-0114 awarded by the Office of Naval Research (BMDO), (ii) F49620-93-C-0018 awarded by AFOSR (DARPA), and (iii) DMR-9115680 awarded by the National Science Foundation, and the Government has certain rights in the invention.
In etching we remove a film or layer from a substrate, in some instances defining the layer to be removed by photolithography. One way to etch is to immerse the substrate in a bath of some chemical that attacks the film. Preferably, the chemical should react with and etch the film or layer in a smooth and reproducible manner, producing soluble products that can be carried away from the substrate. In particular, an ideal etchant will not attack any layer underneath the film being etched, so that the etch process will be self-limiting. Unfortunately, the etching is often not self-limiting and therefore goes below the desired depth. Etch-stops are designed to address this problem.
Semiconductors have the interesting property that when they are alloyed with certain elements, the rate of wet chemical etch in the alloy will vary from that of the unalloyed semiconductor. Alloys with different etch rates can be used to cause etching to slow at a pre-defined interface. Typically, a layer with a particular composition etches at a known rate in an etchant A second adjacent layer may etch at a different rate because it has a different composition. The layer with the lower etch rate is often referred to as the etch-stop layer.
Etch-stops are used to fabricate devices for a wide variety of applications. Membranes and diaphragms formed via etch-stops are used in sensors such as pressure transducers, as elements in experimental x-ray lithography systems, as windows for high energy radiation, and as low thermal mass supports for microcalorimeter and bolometric radiation detectors. Additional uses for selective etch-stops are in micromachining applications such as accelerometers, gears, micro-beams, miniature fluid lines, pumps and valves, and in flow sensors. Another application with a potentially large commercial market is in fabricating silicon-on-insulator (SOI) substrates by the bond-and-etch-back silicon-on-insulator (BESOI) process.
Silicon-based selective chemical etch-stop layers such as Si--B, Si--Ge, Si--Ge--B, Si--P, and Si--As have major problems and disadvantages which are overcome by the present invention. The disadvantages can be illustrated by examining examples pertaining to the commonly used Si--Ge--B etch-stops. First, the specially doped layer (e.g., Si--Ge--B) and the lightly doped silicon have limited selectivity. Selectivity is defined as the etch rate of lightly doped silicon divided by the etch rate of the etch-stop layer, or in some cases its reciprocal as discussed below. Limited selectivity increases the manufacturing cost by creating a need for tightly controlled, and sometimes labor-intensive processing to prevent the etch from going beyond the intended depth. This problem is exacerbated when fabricating the thin layers that are required for submicron electronic devices.
Certain chemical solutions etch a lightly doped silicon layer more rapidly than a heavily doped layer. For this purpose lightly doped means less than approximately 1E17 dopant atoms per cm.sup.3, and heavily doped means more than approximately 1E19 dopant atoms per cm.sup.3. For example, 21 weight percent (wt %) potassium hydroxide in H.sub.2 O (KOH--H.sub.2 O) at about 70.degree. C. etches the (100) plane of lightly doped silicon rapidly (approximately 1 micrometer per minute), but the etch rate becomes slow (less than 0.01 micrometer per minute), making possible selective etching, as the boron concentration in the silicon increases to more than about 5E19 atoms per cm.sup.3.
The conventional formulations of these etchants have serious problems when used with the above-mentioned etch-stop layers. For example, KOH--H.sub.2 0, an inexpensive etchant, is prone to producing a rough surface when it etches silicon. Ethylenediamine pyrocatechol in water (EDP--H.sub.2 O) provides somewhat better etch selectivity and is less prone to developing surface roughness than KOH--H.sub.2 O, but has limited application in that it emits extremely toxic vapors and is relatively expensive. Another etchant, cesium hydroxide in water (CsOH--H.sub.2 O), can provide smoother surfaces than KOH--H.sub.2 O for conventional etch-stop layers but is even more expensive than EDP--H.sub.2 O.
Surface roughness arises from the anisotropic etch properties of the solutions that preferentially etch lightly doped silicon. These solutions etch certain crystallographic directions in the material faster than other directions. For example, a chemical solution consisting of 21 wt % KOH--H.sub.2 O at 70.degree. C. will rapidly etch the (100) plane of lightly doped silicon, but only slowly etch the (111) plane. This leads to the etched surface being rough as illustrated in FIGS. 1A-B. As shown in FIG. 1A, a solution of KOH--H.sub.2 O (11) will rapidly etch lightly doped silicon (12). If a small particle such as particle (13) adheres to the surface of the lightly doped silicon (12), the etch rate will be locally retarded under the particle (13). Slow etching planes (14) on the (111) plane will form as the particle (13) is undercut by the etch solution. This leads to the formation of a slow-etching pyramid under the particle (13). If the etch selectivity is not sufficiently high, these pyramids will propagate into the etch-stop layer (15), shown in FIG. 1A as peaks (16), resulting in a rough surface.
Another problem with conventional etch-stop compositions, especially those containing a high boron concentration, is they leave an insoluble staining residue on the surface of the substrate. This residue both roughens and contaminates the substrate surface. Increasing the KOH concentration of the etch solution, for example, from 21 wt % to 40 wt % will eliminate the surface staining, but will also substantially decrease the etch selectivity.
In contrast to the above etchants, conventional formulations of 1:3:8 and 1:3:12 parts by volume of HF--NHO.sub.3 --CH.sub.3 COOH (HNA) etch lightly doped silicon somewhat less rapidly than heavily doped silicon and are therefore used preferentially to remove etch-stop layers. In this case, selectivity is defined as the etch rate of the etch-stop layer divided by the etch rate of the lightly doped silicon. The above formulations of HNA have major drawbacks, including relatively low selectivity, and selectivity decreasing rapidly with time during etching due to reduction of the HNO.sub.3 to HNO.sub.2.
Still another problem with conventional etch-stop compositions is that the impurity which provides the etch selectivity is also a donor or acceptor dopant in the silicon. Thus, for example, when a Si--Ge--B etch-stop is used to fabricate a BESOI substrate, boron diffusing out from the etch-stop layer during a bonding anneal causes unwanted electrically active dopant in the device layer. This problem is illustrated in FIGS. 2A-B which show concentration profiles of boron and germanium as a function of depth in a substrate layer, an etch-stop layer, and a device layer, before a bonding anneal (FIG. 2A) and after the anneal (FIG. 2B). FIG. 2A illustrates the boron (21) and germanium (22) concentration profiles in the substrate (23), the etch-stop layer (24), and in the device layer (25) after epitaxial layer growth and before the bonding anneal. FIG. 2B shows the changed boron (26) and germanium (27) concentration profiles after the bonding anneal. Because boron diffuses through the material faster than germanium during the bonding anneal, its profile is broadened such that significant "diffusion tails" extend from the etch-stop layer into the substrate (23) and the device layer (25). In a BESOI structure, the boron diffusion tail causes an unacceptable level of electrically active dopant to exist in the device layer (25).
There are reports in the literature of Si--Ge--C layer fabrication. However, to the best of applicants' knowledge, none of the existing processes for forming Si--Ge--C are suitable for producing Si--Ge--C layers as part of a large scale manufacturing process. Feijoo et al., Etch Stop Barriers in Silicon Produced by Ion Implantation of Electrically Non-Active Species, Journal of the Electrochemical Society (1992) describe silicon layers implanted with silicon, germanium, and carbon at doses between 1E14 and 3E16 ions/cm.sup.2 and energies between 35 and 200 keV and testing them as etch-stop barriers in an EDP--H.sub.2 O based solution (p. 2309, Abstract). When ions are implanted in this range of dose and energies, the lattice structure is damaged. Feijoo states the results obtained indicate that the effectiveness of the etch-stop is influenced (i.e., improved) by both the implantation damage and the chemical interaction between the implanted ions and the defective crystal (Abstract). The resulting damage greatly restricts the number of useful commercial applications for Feijoo's etch-stop barriers. Accordingly, Feijoo's methods and results are substantially different from the present invention.
U.S. Pat. No. 4,885,614 to Furukawa et al., Semiconductor Device with Crystalline Silicon-Germanium-Carbon Alloy, describes another process of producing a silicon-germanium-carbon alloy film principally by molecular beam epitaxy, but also by plasma enhanced chemical vapor deposition (CVD), photoenhanced CVD, microwave-excited CVD, thermal CVD and metal-organic CVD methods. Molecular beam epitaxy (MBE) might provide good crystalline quality, but it is a slow and expensive process. With regard to a description of the thermal CVD process (col. 10, lines 37-43) Furukawa describes that the surface of a silicon substrate was cleaned and the temperature thereof adjusted to 650.degree. C. Gaseous SiH.sub.4, GeH.sub.4 and CH.sub.4 were allegedly introduced into a reactor so as to give a total pressure of 100 torr. Thus, a Si--Ge--C alloy film was purportedly formed on the substrate by a thermal CVD method. However, applicants believe methane at the stated process temperature is far too stable to function as a carbon source for thermal CVD formation of the silicon-germanium-carbon film. Further, Furukawa et al. do not recognize or discuss the use of Si--Ge--C as an etch-stop.
Regolini et al., Growth and characterization of strain compensated Si.sub.1,xy Ge.sub.x C.sub.y epitaxial layers, Materials Letters (1993) describe metal-organic chemical vapor deposition (MOCVD) for fabricating epitaxial Si--Ge--C layers with less than 1 atomic percent carbon, which is far less than desirable for etch-stops. Further, there is no mention in the Regolini et al. publication of using Si--Ge--C as an etch-stop.